Major modern diseases such as Ebola virus disease and influenza are zoonoses. Zoonoses can be caused by a range of disease pathogens such as viruses, bacteria, fungi and parasites; of 1,415 pathogens known to infect humans, 61% were zoonotic.[3] Most human diseases originated in animals, however, only diseases that routinely involve animal to human transmission, like rabies, are considered as zoonoses.[4]

Zoonoses have different modes of transmission. In direct zoonosis the disease is directly transmitted from animals to humans through media such as air (influenza) or through bites and saliva (Rabies).[5] In contrast, transmission can also occur via an intermediate species (referred to as a vector), which carry the disease pathogen without getting infected. When humans infect other animals; it is called reverse zoonosis or anthroponosis.[6]

During most of human prehistory groups of hunter-gatherers usually numbered fewer than 150 individuals. Such groups seldom made contact with other bands. Such isolation commonly caused epidemic diseases to burn out after their first run through a population, because epidemics depend on constant contact with humans who have not developed an immune response. To persist in such a population, a pathogen either had to be a chronic infection, staying alive and infectious in the host for long periods, or it had to have a non-human reservoir in which it could survive until new hosts made contact. In fact, for many 'human' diseases, the human is actually an accidental or incidental victim and a dead-end host. Examples include rabies, anthrax, tularemia and West Nile virus. Thus, much of human exposure to infectious disease has been zoonotic.

Many modern diseases, even epidemic diseases, started out as zoonotic diseases. It is hard to establish with certainty which diseases jumped from other animals to humans, but there is increasing evidence from DNA and RNAsequencing, that measles, smallpox, influenza, HIV, and diphtheria came to us this way. Various forms of the so-called "common cold" and tuberculosis also are adaptations of strains originating in other species.

A major factor contributing to the appearance of new zoonotic pathogens in human populations is increased contact between humans and wildlife.[8] This can be caused either by encroachment of human activity into wilderness areas or by movement of wild animals into areas of human activity. An example of this is the outbreak of Nipah virus in peninsular Malaysia in 1999, when intensive pig farming began on the habitat of infected fruit bats. Unidentified infection of the pigs amplified the force of infection, eventually transmitting the virus to farmers and causing 105 human deaths.[9]

Similarly, in recent times avian influenza and West Nile virus have spilled over into human populations probably due to interactions between the carrier host and domestic animals. Highly mobile animals such as bats and birds may present a greater risk of zoonotic transmission than other animals due to the ease with which they can move into areas of human habitation.

In 2006, a conference held in Berlin was focusing on the issue of zoonotic pathogen effects on food safety, urging governments to intervene, and the public to be vigilant towards the risks of catching food-borne diseases from farm-to-dining table.[13]

Many food outbreaks can be linked to zoonotic pathogens. Many different types of food can be contaminated that have an animal origin. Some common foods linked to zoonotic contaminations include eggs, seafood,meat, dairy, and even some vegetables.[14] Food outbreaks should be handled in preparedness plans to prevent widespread outbreaks and to efficiently and effectively contain outbreaks.

In 1988, a person became ill with swine influenza virus (swine flu) and died after visiting the display area of the pig barn at a Wisconsin county fair. Three healthcare personnel treating the case patient also developed flu-like illness with laboratory evidence of swine influenza virus infection.[17] Investigators from the CDC indicated in their final report that the swine flu had been transmitted directly from pig to human host.[18]

In 1994, seven cases of E. coli O157:H7 infection were traced to a farm in Leicestershire, United Kingdom. An epidemiological investigation into the outbreak revealed that the strain of E. coli O157:H7 isolated from nine animals on the farm was indistinguishable from the strain isolated from human samples. Investigators concluded that the most likely cause of this outbreak was direct human contact with animals.[19]

In 1995, 43 children who had visited a rural farm in Wales became ill with Cryptosporidiosis. Cryptosporidium was isolated from seven of the ill children. An epidemiological investigation indicated that the source of the children's illness was contact with calves at the farm.[20]

Also in 1995, at least 13 children became ill with Cryptosporidiosis after visiting a farm in Dublin, Ireland. In a case-control study, researchers compared the activities of the 13 ill children, or cases, to the activities of 52 out of 55 people who had visited the farm – the controls. The study revealed that illness was significantly associated with playing in the sand in a picnic area beside a stream where animals had access.[21]

In 1997, an E. coli O157:H7 outbreak was identified among one child who lived on an open farm and two children who visited the farm during school parties. Two of the three children developed hemolytic-uremic syndrome (HUS). Isolates collected from the three children and from samples taken at the farm were indistinguishable, demonstrating evidence of the link between the farm and the children's illness.[22]

In 1999, what is believed to be the largest outbreak of waterborne E. coli O157:H7 illness in United States history occurred at the Washington County, New York fair. The New York State Department of Health identified 781 individuals who were suspected of being infected with either E. coli O157:H7 or Campylobacter jejuni. An investigation into the outbreak revealed that consumption of beverages purchased from vendors supplied with water drawn from an unchlorinated fairgrounds well was associated with illness. In all, 127 outbreak victims were confirmed ill with E. coli O157:H7 infections; 71 were hospitalized, 14 developed HUS, and two died.[23]

In 2000, 51 people became ill with confirmed or suspected E. coli O157:H7 infections after visiting a dairy farm in Pennsylvania. Eight children developed HUS. A case-control study among visitors to the dairy was conducted jointly by the CDC, Pennsylvania Department of Health, and the Montgomery County Health Department. The study's authors concluded that E. coli was transmitted to visitors as a result of contamination on animal hides and in the environment.[24]

Also in 2000, 43 visitors to the Medina Countyfair in Ohio were confirmed ill with E. coli O157:H7 infections. An investigation into the outbreak suggested that the water system from which food vendors were supplied was the source of the E. coli outbreak. Several months later, five children became ill with E. coli infections after attending a "Carnival of Horrors" event held at the Medina County fairgrounds. PFGE analysis of the strains of E. coli isolated from members of both outbreaks revealed an indistinguishable pattern, and investigators from the Medina County Health Department and the CDC determined that the Medina County Fairgrounds water distribution system was the source of both E. coli outbreaks.[25]

In 2001, an E. coli O157:H7 outbreak was traced to exposure in the Cow Palace at the Lorain County Fair in Ohio. CDC investigators identified 23 cases of E. coli infection associated with attendance at the Lorain County Fair, with additional secondary cases likely. Two people developed HUS. An investigation revealed E. coli contamination on doorways, rails, bleachers, and sawdust. Investigators concluded that the Lorain County Fair was the source of the outbreak.[26]

Wyandot County, Ohio, also reported an E. coli O157:H7 outbreak in 2001. Ninety-two E. coli infections were reported to the Wyandot County Health Department and the CDC, with 27 cases confirmed using laboratory analysis. Two cases developed HUS. Contact with infected cattle was believed to be the source of the outbreak; however, a specific cause was never identified.[26]

In 2002, 75,000 people became ill with E. coli O157:H7 infections after visiting a large agricultural fair in Ontario, Canada. Outbreak investigators conducted a case-control study, which indicated that goats and sheep from a petting zoo were the source of the E. coli among fair visitors. Other indications were that the fencing and environment surrounding the petting zoo could have been a source of transmission.[27]

What is believed to be the largest E. coli O157:H7 outbreak in Oregon history occurred among attendees at the Lane County fair in 2002.[28] An Oregon Department of Human Services – Health Services investigation led to the belief that the E. coli outbreak originated from exposure in the sheep and goat barn. In all, 79 people were confirmed ill with E. coli infections as part of the outbreak; 22 were hospitalized, and 12 suffered HUS.[29]

In 2003, fair visitors and animal exhibitors at the Fort Bend County Fair in Texas became ill with E. coli O157:H7 infections. An outbreak investigation led to the determination that 25 people had become ill with E. coli infections after attending the Fort Bend County Fair; seven people were laboratory-confirmed with E. coli, and 5 developed HUS or TTP (Thrombotic thrombocytopenic purpura). Investigators isolated a strain of E. coli indistinguishable from the outbreak strain from four animal husbandry sites, and found high levels of E. coli contamination in both rodeo and animal exhibit areas.[30]

In 2004, a large E. coli O157:H7 outbreak occurred among visitors at the 2004 North Carolina State Fair. During its investigation into the outbreak, the North Carolina Department of Health and Human Services (NCDHHS) received over 180 reports of illness, and documented 33 culture-confirmed cases of E. coli O157:H7 associated with attendance at the fair, with 15 children developing HUS. In its final investigation report, NCDHHS concluded that the North Carolina State Fair E. coli outbreak had originated at a petting zoo exhibit. The conclusion was supported by a case-control study, environmental sampling, and laboratory analysis of samples collected from the fair and members of the outbreak.[31]

In 2005, a petting zoo that exhibited at two Florida fairs and a festival was traced as the source of an E. coli O157:H7 outbreak. Sixty-three people who had visited either the Florida State Fair, the Central Florida Fair, or the Florida Strawberry Festival reported illness to investigators for the Florida Department of Health, including 20 who were culture-confirmed and 7 with HUS. A case-control study revealed that illness was associated with exposure to a petting zoo exhibit present at all three events.[32]

^Marx PA, Apetrei C, Drucker E (October 2004). "AIDS as a zoonosis? Confusion over the origin of the virus and the origin of the epidemics.". Journal of medical primatology33 (5-6): 220–6. doi:10.1111/j.1600-0684.2004.00078.x. PMID15525322.

^Goode, B.; O'Reilly, C. (29 June 2005). "Outbreak of Shiga toxin producing E. coli (STEC) infections associated with a petting zoo at the North Carolina State Fair – Raleigh, North Carolina, November 2004 Final Report". North Carolina Department of Health and Human Services.

In its chapters on domestic pets, the book – Zoonosis: Infections Affecting Humans and Animals – lists a chilling selection of unpleasant illnesses people can pick up from cats and dogs. These include a wide range of gut-nasties, staphylococcus, ...

Addressing the conference, Dr Uday Kakroo, Director, MIEF, said that outbreaks of diseases communicable from animals to humans are commonly referred to as Zoonosis. These have been traced to human interaction with and exposure to animals at fairs, ...

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